Interrogation method for passive sensor monitoring system

ABSTRACT

A method of determining the frequency of a plurality of resonant devices (for example three SAW devices) includes determining optimal interrogation frequencies for each of the devices, the optimal interrogation frequencies having maximum power spectral densities, accumulating a plurality of responses for each sensor, performing discreet Fourier transforms on the sampling results to estimate the three resonant frequencies, and averaging the results of the Fourier transforms to provide an indication of resonant frequencies. The averaging step may include the calculation of a standard deviation and the rejection of any results which fall more than a predetermined multiple of the standard deviation from the average frequency result. The frequency is determined by the method may be employed to calculate the pressure and temperature of the sensor devices. The sensor devices may be located in a vehicle tire.

TECHNICAL FIELD OF THE INVENTION

[0001] This invention relates to a method for interrogating sensorsystems based on wirelessly interrogated passive sensory-transponders asused, for example, for measuring pressure and temperature of air invehicle tires. More specifically, the preferred embodiment of theinvention provides a passive sensor interrogation algorithm which allowshigh accuracy of measurement of pressure and temperature.

DESCRIPTION OF RELATED ART

[0002] A number of solutions for the problem of a wireless interrogationof passive pressure and temperature sensors are known in the prior art.The sensors utilize either one-port delay lines or one-port resonators,preferably based on SAW technology, although other approaches are alsopossible (bulk acoustic wave devices or dielectric resonators, forinstance). The use of the delay lines [see F. Schmidt and G. Scholl,Wireless SAW identification and sensor systems. In a book “Advances inSurface Acoustic Wave Technology, Systems and Applications”. Ed. C. W.C. Ruppel and T. A. Fjeldly, Singapore, World Scientific, 2001, p. 287.]or the resonators [see W. Buff, S. Klett, M. Rusko, J. Ehrenpfordt, andM. Goroll. Passive remote sensing for temperature and pressure using SAWresonator devices. IEEE Trans. On Ultrasonics, Ferroelectrics, andFrequency Control, vol. 45, No. 5, 1998, pp. 1388-1392.] is dictated bya necessity to distinguish a passive sensor response on the one hand anda direct feed-through signal together with environmental echo signals onthe other hand. This is achieved by employing the fact that the impulseresponse of the delay lines and the resonators is considerably longerthan any parasitic signal.

[0003] Interrogation of passive SAW sensors based on delay lines isusually performed by means of very short (typically, 0.1 μs) RF pulses.As a result, the interrogation system requires relatively wide bandwidthof 10 MHz or even more, which is not available in license-freeindustrial-scientific-medical (ISM) bands below 1 GHz. Sensors based onhigh Q-factor one-port resonators are more appropriate for these bandsdue to their narrowband response. For this reason, we shall concentrateon interrogation of resonator-type passive sensors, preferably based onSAW resonators. The main purpose of the interrogation is to measure thefrequency of natural oscillations in the resonators (resonantfrequencies) excited by relatively long and narrowband RF interrogationpulses. Since the resonant frequencies can be made to depend ontemperature and pressure, knowing the resonant frequencies allowstemperature and pressure to be calculated.

[0004] In order to exclude influence of varying antenna impedance on theresonant frequency the prior art [see W Buff, S. Klett, M, Rusko, J.Ehrenpfordt, and M. Goroll mentioned above] proposes that a differencebetween the frequencies of natural oscillations of two similarresonators (possibly with slightly different resonant frequencies)connected to one antenna, is measured. If both resonators are at thesame temperature and have different pressure sensitivity, the pressurecan be found from the frequency difference and the influence oftemperature will be greatly reduced. The two resonators can be veryefficiently interrogated by bi-harmonic RF pulse exciting naturaloscillations in both resonators simultaneously [see GB9925538.2]. Whenthe interrogation pulse is over the response will present anexponentially decaying beat signal with the beat frequency equal to themeasured frequency difference. The beat frequency can be accuratelydetermined by means of amplitude detection and period count.

[0005] In the case of simultaneous measurement of both pressure andtemperature, at least three resonators connected to one antenna arerequired and two frequency differences need to be measured in order tocalculate the two unknowns, the pressure and the temperature [see W.Buff, M. Rusko, M. Goroll, J. Ehrenpfordt, and T. Vandahl. Universalpressure and temperature SAW sensor for wireless applications. 1997 IEEEUltrasonics Symp. Proceedings, 1997, pp. 359-362]. Measuring the beatfrequency is impossible in this case. The following interrogationtechniques are known from literature.

[0006] 1. The resonators are excited by RF pulses in turn. Theexponentially decaying response of each resonator is picked up byantenna is used as an input signal for a gated PLL tracking variationsof the resonant frequency [see A. Pohl, G. Ostermayer, and F. Seifert.Wireless sensing using oscillator circuits locked to remote high-Q SAWresonator. IEEE Trans. On Ultrasonics, Ferroelectrics, and FrequencyControl, vol. 45, No. 5, 1998, pp. 1161-1168]. This technique is moreappropriate for a single resonator and becomes too cumbersome andunreliable in the case of three resonators, especially if theirfrequencies are close to each other.

[0007] 2. The resonators are excited by RF pulses in turn. Theexponentially decaying response of each resonator is picked up byantenna is down-converted to a lower intermediate frequency and then theperiod of the natural oscillation is counted [see GB9925538.2]. Thismethod also works well either for a single resonator or when thedistance between the resonant frequencies is much larger than theresonator bandwidth. However, if it is less than 10 times the bandwidth(which is the case in ISM band), then more than one resonator will beexcited by the RF pulse causing parasitic frequency modulation in thesensor response and drastically reducing accuracy of measurement.

[0008] 3. All three resonators are excited in one go. The spectrum ofthe sensor response is analyzed in the receiver by means of discreteFourier transform and all resonant frequencies are measured [see L.Reindl, G. Scholl, T. Ostertag, H. Scherr, and F. Schmidt. Theory andapplication of passive SAW radio transponders as sensors. IEEE Trans. OnUltrasonics, Ferroelectrics, and Frequency Control, vol. 45, No. 5,1998, pp. 1281-1291]. This approach allows interrogation of a largenumber of resonators. However it requires the use of a broadband RFpulse covering the whole frequency range of operation of the sensor.Bearing in mind that the peak power of the interrogation pulse islimited in ISM band (usually it is not more than 10 mW) it is clear thatspreading the spectrum of the pulse reduces efficiency of resonatorexcitation. It adversely affects signal-to-noise ratio (SNR) and henceaccuracy of measurements.

SUMMARY OF THE INVENTION

[0009] The object of the present invention is to provide aninterrogation method that preserves the advantages of Fourier analysisand at the same time provides high efficiency of resonator excitationand high accuracy of measurements.

[0010] In accordance with one aspect of the present invention a methodof interrogating a plurality of resonant devices to determine therespective resonant frequencies of the devices comprises the steps of:

[0011] (1) determining, for each resonant device, an optimalinterrogation frequency;

[0012] (2) repeat the interrogation of each resonant device a pluralityof times at its respective optimal interrogation frequency as determinedby step (1)

[0013] (3) perform discreet Fourier transforms on the data accumulatedas a result of step (2) and

[0014] (4) determine the average of the frequencies derived from step(3).

[0015] The invention would be better understood from the followingdescription of a preferred embodiment thereof, given by way of exampleonly, reference being had to the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 illustrates schematically a pressure and temperaturemonitoring system for use in a vehicle tire; and

[0017]FIG. 2 illustrates the interrogation algorithm proposed by thepresent invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0018] Referring firstly to FIG. 1, the present invention isparticularly applicable to a system for monitoring the temperature andpressure in a vehicle tire. However, it is to be understood that theinvention is not limited to this application and may be applied to othercircumstances where pressure and temperature are to be monitored, orindeed to other circumstances where a plurality of other perimeters areto be measured by a passive sensor system. The preferred embodiment ofthe present invention includes three surface acoustic wave devices SAW1,SAW2 and SAW3 which are connected to a common antenna 12. Whilst the useof SAW devices is preferred as a means of generating signals indicativeof the sensed condition, it is to be understood that the invention isnot limited to such devices and other passive sensors capable ofproviding appropriate indications by means of resonant frequency may beemployed.

[0019] In the particular preferred application of the present invention(vehicle tire pressure and temperature sensing) the SAW devices SAW1,SAW2, SAW3 and the antenna 12 are mounted as a unit A within a vehicletire. An excitation and monitoring unit B is located on the vehicle inorder to provide excitation signals to the tire mounted unit and toreceive response signals from it. For this purpose, the unit B includesan antenna 11 for communicating with the antenna 12 of the package A.

[0020] An interrogation pulse is generated by a power amplifier 8 thatis excited by a transmitter synthesiser 10. The pulse goes through an RFswitch 1 to the antenna 11 of the interrogation unit B. The radiatedelectromagnetic wave is picked up by the antenna 12 of the sensor unit Athus exciting the three SAW resonators in the sensor. Re-radiated sensorresponse is transmitted by the sensor antenna and received by theantenna 11. The signal goes through a front-end low-noise amplifier 2 tothe frequency converter where it is mixed with the signal of thereceiver synthesizer 3. The frequency difference between the receiversynthesizer 3 and the transmitter synthesizer 10 equal to theintermediate frequency, e.g. 1 MHz. The IF signal goes through a filter4 and a limiting amplifier (which increases the dynamic range of thereceiver) to an 8-bit or 10-bit analog-to-digital converter 6 with thesampling rate sufficiently high in comparison with the IF, e.g. 10 or 20MHz. The sensor response in digital format is stored in the internalmemory of a DSP chip 7 where it is accumulated in a coherent way duringthe interrogation process. The chip then performs Fourier transformationof the data for all three SAW resonators, calculates three resonantfrequencies, performs averaging procedure and calculates the pressureand the temperature. The DSP chip 7 also controls the operation of thesynthesizers 3 and 10, RF switch 1, and the ADC 6. Besides, it can alsoenable and disable the power amplifier 8 and the LNA 2 in order toincrease an isolation between the receiver and the transmitter. As oneof the measures to ensure coherent accumulation of the sensor responsesthe same quartz crystal oscillator 9 is preferably used as a referencefor both synthesizers and for the DSP chip.

[0021] The above system can also be implemented by using a doublefrequency conversion receiver that increases image channel rejection. Analternative receiver architecture can be based on a direct frequencyconversion. This would cause the removal of one of the synthesizers andaddition of the second mixer and ADC to produce a quadrature channel.

[0022] Referring now to FIG. 2, the preferred method of the presentinvention will be described. The three resonators SAW1, SAW2, SAW3 haveslightly different resonant frequencies and different temperature andpressure sensitivities. The frequencies are chosen in such a way thatthe minimum distance between them is not less than the resonatorbandwidth at any pressure and temperature. As a result, the wholeoperational frequency band (ISM band, for instance) is divided intothree sub-bands occupied by the three resonators.

[0023] The sensor A is interrogated by rectangular RF pulses with thespectral width equal to or less than the resonator bandwidth. Thisensures efficient excitation of the resonator in the case if theinterrogation frequency is close to the resonant frequency of theresonator. In each sub-band, there are several discrete interrogationfrequencies chosen in such a way that the distance between them is equalto or less then the bandwidth of the resonators. The number of thediscrete interrogation frequencies depends on the Q-factor of the SAWresonators. For instance, in the case of the unloaded Q=5000 it would beenough to have nine interrogation frequencies within the 434 MHz ISMband.

[0024] As a result, whatever the temperature and pressure is, there willalways exist three interrogation frequencies from the set of the chosendiscrete frequencies that ensure optimal excitation of the threeresonators. The excitation is optimal in the sense that the amplitude ofoscillation in the resonator will be close to maximum possible one for agiven excitation amplitude by the end of the interrogation pulse.

[0025] The interrogation procedure consists of five main stages asillustrated by the flowchart in FIG. 2.

[0026] 1. Determination of the Three Optimal Interrogation FrequenciesMaximizing Power Spectral Density of the Sensor Response.

[0027] At this stage, the sensor is interrogated at all discreteinterrogation frequencies one after another. Each time, after launchingthe interrogation pulse, the sensor response is received and itsspectral density is found. It can be done, for instance, by frequencydown-conversion, sampling the response at the intermediate frequency andcalculating discrete Fourier transform. After that, three optimalfrequencies are chosen, one in each sub-band, giving maximum peak valueof the spectral density. Alternatively, if a linear amplifier withautomatic gain control is used in the receiver, the three frequenciescan be chosen that maximize the ratio of the peak value of the spectraldensity to the average level of its sidelobes. Alternatively, if thelimiting amplifier is used in the receiver, the three frequencies can bechosen that maximize the length of the sensor response. At this stage wecan already determine the three resonant frequencies by measuring thepeak frequencies of the spectral density. However, this would give usjust a rough estimate of the actual frequencies of natural oscillationsbecause of the presence of noise and finite resolution of Fourieranalysis.

[0028] 2. Coherent Accumulation of Sensor Responses

[0029] At this stage we repeat interrogation of the sensor N times ateach optimal interrogation frequency in turn. The signals picked up bythe receiver are down-converted, sampled and accumulated in a coherentway in three data arrays in a system memory. The aim of the coherentaccumulation is to increase SNR by a factor of N. Coherent accumulationcan be ensured, for instance, by using a common quartz-stabilizedoscillator both in receiver and transmitter synthesizers and as a clockgenerator in the DSP chip. In other words, the period of theinterrogation signal at the intermediate frequency and the distancebetween the interrogation pulses are chosen to be an integer number ofthe sampling period. Besides, the number of accumulated pulses N ischosen to be sufficiently small (N=10 . . . 30) so that the total timeneeded for coherent accumulation (approximately 1 . . . 2 ms) is smallenough ({fraction (1/40)}, for instance) in comparison with the periodof a vehicle tire rotation. As a result, a change in a position of thesensor antenna will not cause a large variation in the phase of thesensor response during accumulation. It is also important from the pointof view of minimizing a variation of the frequency differences betweenthe three resonators caused by the antenna impedance variation as aresult of the tire rotation.

[0030] Before doing coherent accumulation the presence of interferenceis also checked at each of the three optimal interrogation frequencies.This can be done for instance by comparing maximum of the spectraldensity of the signal received in the absence of the interrogation pulsewith an appropriate threshold level. If it exceeds the threshold levelthen the system repeats interrogation after some delay. A simplerinterference detection procedure can also be used within the coherentaccumulation cycle. In this case, the interference can be detected bymeasuring the peak value of the received signal during 1-2 μs beforelaunching each interrogation pulse.

[0031] 3. Discrete Fourier Transform and Interpolation

[0032] At this stage the three data arrays obtained as a result of thecoherent accumulation are used to calculate three spectral densities bymeans of discrete Fourier transformation (DFT). Each spectrum contains apeak corresponding to the frequency response of a single resonatoralthough there may be other peaks due to excitation of the two otherresonators. However, the main peak has larger amplitude and smallerpeaks are disregarded. The main peak frequency corresponds to therelevant frequency of the natural oscillation. Resolution of Fourieranalysis Δf is increased by zero filling so that the analysis time isincreased, for instance, from 10-20 μs up to 0.1-0.2 ms giving Δf=5-10kHz. This accuracy is not still sufficient for many applications.

[0033] Further increase in accuracy is achieved by using quadratic orhigher-order interpolation in the vicinity of the peak frequency inorder to accurately find the resonant frequency for each threeresonators. As a result, the accuracy is no longer limited by theresolution of Fourier analysis but mainly by the system noise.

[0034] Apart from the random component of the frequency measurementerror due to noise there is also a systematic component (bias) due tothe finite length of the sensor response. The value of the bias dependson the initial phase angle of the sensor response pulse at theintermediate frequency and it can vary from one cycle of coherentaccumulation to another one. It is impossible to predict it because theinitial phase is determined by the distance between the unknown resonantfrequency and the interrogation frequency. The following methods areused to reduce considerably the bias and hence increase the accuracy ofthe system.

[0035] a) Coherent accumulation is repeated twice at each interrogationfrequency but an additional phase shift of 90° is introduced into theinterrogation pulse during the second cycle of accumulation.Alternatively, the samples are taken with the delay τ=1/(4f_(int)) wheref_(int) is the nominal intermediate frequency (the difference betweenthe interrogation frequency and the frequency of the local oscillator)during the second cycle of accumulation. DFT and interpolationprocedures are also performed twice and the average of the two resultingpeak frequencies is found. This average frequency becomes much closer tothe measured resonant frequency since the biases in the two peakfrequencies have opposite signs and approximately equal absolute valuesand they cancel each other. A disadvantage of this method is that theoverall time of measurement is doubled.

[0036] b) The second method does not require an increase of the time ofmeasurement. Coherent accumulation is repeated once at eachinterrogation frequency. The sampling rate is chosen in such a way thatthe sampling interval T_(s) corresponds to 90° phase shift at thenominal intermediate frequency divided by any integer. In other words,T_(s)=τ/n where n=1,2,3 . . . . For instance, if f_(int)=1 MHz thenT_(s) can be chosen to be equal to 0.05 μs since τ=0.25 μs. The firstDFT is performed for the samples starting from the first one and thesecond DFT is performed for samples starting from the n-th one.Effectively it means that we have a 90° phase shift between the two setsof samples. As a result of averaging of the two peak frequencies foundby means of DFT and interpolation the value of the bias is considerablyreduced. As an example, the maximum value of the bias is reduced from1.69 kHz to 0.57 kHz for the minimum distance between the three resonantfrequencies of 350 kHz.

[0037] 4. Statistical Processing and Analysis of the Resonant FrequencyData

[0038] Stages 1 to 3 (or 2 and 3 only if the resonant frequencyvariation is slow and a frequent repetition of stage 1 is not required)are continuously repeated and the data on the three resonant frequenciesare stored in three data arrays in the system memory. After M cycles ofinterrogation (M can vary in a wide range, for instance, from 10 To 300)average values f_(1,2,3) and standard deviations σ_(1,2,3) of each ofthe three resonant frequencies are calculated. As a result, the standarddeviations of f_(1,2,3) are further decreased in comparison withσ_(1,2,3) approximately by a factor of M. Then all the frequencies f_(i)in the relevant arrays not satisfying the condition

|f_(i)-f_(1,2,3)| kσ_(1,2,3)

[0039] (where k may have values from 1 to 3) are excluded fromconsideration and the average frequencies are re-calculated again. Thelast procedure is performed in order to exclude possible influence ofinterference and sudden decrease in the signal amplitude during coherentaccumulation causing rough errors in the resonant frequencies. Thestandard deviations σ_(1,2,3) can also be used as a measure of validityof the information about the resonant frequencies.

[0040] 5. Calculation of Pressure and Temperature

[0041] After averaging two difference frequencies are calculated andthen the pressure and the temperature arc found using, for instance,approach described in Ref. [4].

[0042] The proposed interrogation method is aimed to achieve theaccuracy of the resonant frequency measurement better than 5×10⁻⁶. Inthe case of SAW resonators working in 434 MHz ISM band it should givethe accuracy of pressure measurement better that 1 psi and the accuracyof temperature measurement better than 1° C.

1. A method of interrogating a plurality of resonant devices todetermine the respective resonant frequencies of the devices comprisingthe steps of: (1) determining, for each resonant device, an optimalinterrogation frequency; (2) repeating the interrogation of eachresonant device a plurality of times at its respective optimalinterrogation frequency as determined by step (1); (3) performingspectrum estimations on the data accumulated as a result of step (2);and (4) determining the average of the frequencies derived from step(3).
 2. The method according to claim 1 wherein step (4) furtherincludes the steps of determining the standard deviation for each of theaverage frequencies determined; rejecting any frequencies which varyfrom the average frequency by more than a predetermined multiple of thestandard deviation; and re-calculating the average frequency afterexclusion of the rejected data.
 3. The method according to claim 1,wherein the optimal interrogation frequencies are determined byestablishing the frequencies at which the signals from the resonantdevices have a maximum power spectral density.
 4. The method accordingto claim 4, wherein the maximum power spectral density is determined byfrequency down-conversion, sampling the response at an intermediatefrequency, and calculating discreet Fourier transforms.
 5. The methodaccording to claim 3, wherein the maximum power spectral density isdetermined by means of a linear amplifier with automatic gain control,the optimal frequencies being chosen to maximize the ratio of peak valueof the spectral density to average level of its side lobes.
 6. Themethod according to claim 3, wherein the maximum power spectral densityis determined by use of a limiting amplifier, the frequencies beingchosen to maximize the length of sensor response.
 7. The methodaccording to claim 1, wherein the optimal interrogation frequencies liewithin respective sub-bands of an ISM band.
 8. The method according toclaim 1, wherein during the repetition of step 2 of claim 1 each signalreceived is down-converted, sampled and accumulated to provide acoherent accumulation of optimal interrogation frequencies.
 9. Themethod according to claim 8, wherein the coherent accumulation isachieved by using a common oscillator both in the receiver andtransmitter synthesizers and a clock generator in a DPS chip.
 10. Themethod according to claim 1, wherein the number of repetitiveinterrogations in step 2 and the speed at which such interrogations areconducted is such that the total interrogation period is small incomparison with the period of any cyclical movement of the sensorsrelative to the interrogation equipment.
 11. The method according toclaim 8 wherein each coherent accumulation is repeated at eachinterrogation frequency but with an additional phase shift of 90°introduced into the interrogation pulse during the second cycle ofaccumulation of the samples are taken with the delay during the secondcycle of accumulation.
 12. The method according to claim 4, wherein thesampling rate is chosen in such a way that the sampling intervalcorresponds to a 90° phase shift at the intermediate frequency dividedby an integer.
 13. The method according to claim 1, wherein thedetermined frequencies are used to calculate pressure and temperature.14. The method according to claim 1, wherein the resonant devices areSAW devices.
 15. The method according to claim 1, wherein step (3)includes performing discrete Fourier transforms of the signals obtainedat step (2).
 16. A method of interrogating a plurality of resonantdevices to determine the respective resonant frequencies of the devicescomprising the steps of: (1) determining, for each resonant device, ainterrogation frequency; (2) repeating the interrogation of eachresonant device a plurality of times at its respective interrogationfrequency as determined by step (1); (3) performing spectrum estimationson the data accumulated as a result of step (2); and (4) determining theaverage of the frequencies derived from step (3).
 17. The methodaccording to claim 16, wherein step (4) further includes the steps ofdetermining the standard deviation for each of the average frequenciesdetermined; rejecting any frequencies which vary from the averagefrequency by more than a pre-determined multiple of the standarddeviation; and re-calculating the average frequency after exclusion ofthe rejected data.
 18. The method according to claim 16, wherein theinterrogation frequencies are determined by establishing the frequenciesat which the signals from the resonant devices have a maximum powerspectral density.
 19. The method according to claim 18, wherein themaximum power spectral density is determined by frequencydown-conversion, sampling the response at an intermediate frequency, andcalculating discreet Fourier transforms.
 20. The method according toclaim 18, wherein the maximum power spectral density is determined bymeans of a linear amplifier with automatic gain control, the frequenciesbeing chosen to maximize the ratio of peak value of the spectral densityto average level of its side lobes.